This invention relates to reactor containment facilities and, in particular, to reactor containment facilities improved in terms of the heat dissipation characteristic of a reactor containment vessel.
As disclosed in JP.A.63-75594 and JP.A.63-191096, a reactor containment vessel includes a dry well, which defines a space where a reactor pressure vessel containing a core is arranged, and a suppression chamber. The suppression chamber holds suppression-pool water and defines a wet well in the space above it, with the dry well communicating with the suppression-pool water through vent pipes. The outer periphery of this suppression chamber is surrounded by a steel wall, which constitutes the containment vessel, with the steel wall being surrounded by an outer peripheral pool containing a cooling water that is in contact therewith.
In this reactor containment vessel, the coolant in the reactor pressure vessel, turned into steam that is at high temperature and pressure by being heated by the core, is conveyed from the reactor pressure vessel to the exterior of the reactor containment vessel through pipes. Any rupture in the pipes will cause some of the coolant in the reactor pressure vessel to leak into the dry well as steam at high temperature and pressure to occupy the same (a loss-of-coolant accident); then, the coolant steam will be discharged therefrom, along with the nitrogen with which the dry well has been filled, through the vent pipes into the suppression-pool water, where the steam condenses, with the nitrogen being accumulated in the wet well as noncondensing gas. The transfer of the noncondensing gas from the dry well to the wet well is completed in several minutes after the occurrence of the accident; afterwards, it is only the steam discharged from the reactor pressure vessel that flows into the suppression-pool water. The condensation of this steam causes the temperature of the suppression-pool water to be increased, generating a difference in temperature between the suppression-pool water and the outer-peripheral-pool water. Since the containment-vessel wall separating the suppression chamber from the outer peripheral pool is made of steel, which is a good conductor of heat, the above-mentioned difference in temperature causes the heat held by the suppression-pool water to be transferred to the outer-peripheral-pool water through the wall of the reactor containment vessel. Due to this arrangement, the heat in the reactor containment vessel can be discharged to the exterior thereof over a long period of time after the occurrence of the accident, without using any dynamic apparatus, with the result that a rise in pressure in the reactor containment vessel is suppressed, thereby ensuring the soundness of the reactor containment vessel.
Furthermore, since it promotes the heat dissipation from the reactor containment vessel in a natural manner, without using any dynamic apparatus, the above-described containment vessel is referred to as a natural-heat-dissipation-type or natural-cooling-type containment vessel, which provides a high level of reliability since it employs no dynamic apparatus.
Thus, of those reactor containment vessels endowed with a pressure-rise suppressing function to cope with a loss-of-coolant accident, which is to be taken into account from the viewpoint of safety when designing a nuclear reactor, the natural-heat-dissipation-type containment vessel, which is equipped with a cooling water pool in the outer periphery thereof, can be cooled by transferring heat from the suppression chamber to the outer-peripheral-pool water through the containment-vessel wall, thereby suppressing pressure rise in the containment vessel. When applied to a plant of a relatively large output power, this natural-heat-dissipation-type containment vessel entails, at the time of an accident, an increase in decay heat, which is discharged from the reactor core into the space in the containment vessel; this increase in decay heat is in proportion to the output power, so that it is necessary to proportionately increase the quantity of heat that can be dissipated to the exterior of the containment vessel.
One method of increasing the heat dissipation from the natural-heat-dissipation-type containment vessel is to enlarge the area of the heat transfer surface through which heat is transferred from the suppression chamber to the outer-peripheral-pool water.
In the case where the wall of the reactor containment vessel is used as the heat transfer surface, the heat transfer area can be increased by enlarging the diameter of the containment vessel, or increasing the water depth of the vent pipes so as to attain an enlargement in the height direction of the region which is effective in transferring heat to the outer peripheral pool. Enlarging the diameter of the reactor containment vessel, however, is not desirable since it would entail deterioration in the pressure withstanding capacity of the containment vessel, which would lead to a decrease in the allowable temperature of the suppression chamber, resulting in a degeneration in heat dissipation characteristic. Increasing the water depth of the vent pipes, on the other hand, involves an excessive swell of the suppression-pool water when a great amount of steam rapidly enters the suppression chamber at the initial stage of an accident, so that it is necessary to increase the height of the space above the pool water or augment the strength of the structures inside the suppression chamber. Thus, this method is not desirable, either.
Prior-art techniques for enlarging the heat transfer area without enlarging the diameter of the containment vessel or increasing the water depth of the vent pipes, are disclosed in JP.A.64-91089 and JP.A.2-181696, according to which the outer-peripheral-pool water is circulated through pipes running through the interior of the suppression chamber, thus utilizing the heat dissipation from the pipes running through the suppression chamber as well as the natural heat dissipation through the containment-vessel wall.
Another prior-art technique in this regard was presented in the "Fall Meeting of Atomic Energy Society of Japan in the Year 1989". According to the technique presented, a convection promoting plate is provided in the suppression pool to promote the pool water circulation in the lower region of the suppression pool, thereby mitigating the temperature stratification in the suppression pool; due to this arrangement, that region of the suppression pool which is effective in absorbing the heat from the nuclear reactor and the heat transfer area for heat dissipation can be enlarged in the vertical direction.
According to still another prior-art technique, not only the suppression-pool water but also the wet well is cooled by utilizing the containment-vessel wall; in this prior-art technique, which is shown in JP.A.2-227699, the entire containment vessel is surrounded by a flow passage, through which air is circulated to effect cooling.
The prior-art techniques mentioned above, however, have the following problems:
In the prior-art techniques described in JP.A.64-91089 and JP.A.2-181696, the outer-peripheral-pool water which has been heated to high temperature by the heat released from the suppression-pool water, is allowed to circulate, so that, in the region below the vent-pipe outlets, it is always the temperature on the side of the outer peripheral pool that rises first. As a result, heat transfer takes place in that region from the outer peripheral pool toward the suppression chamber, so that the heat which has been released to the outer peripheral pool in the region above the vent-pipe outlets is again absorbed in the lower region by the suppression chamber. Thus, while an increase in heat reserve can be expected in the region below the vent-pipe outlets, the heat dissipation area for releasing heat to the outer peripheral pool is not increased; on the contrary, it rather decreases. Further, in this prior-art technique, no consideration is given to the continuity of the water circulation in the heat transfer pipes, which circulation is based on the difference in density due to the difference in temperature between the heat transfer pipes and the outer peripheral pool. The water in the heat transfer pipes and that in the outer peripheral pool are heated by the heat released from the suppression chamber and are reduced in density to be accumulated in the upper section of the pool. Since this accumulation takes place both in the heat transfer pipes and in the outer peripheral pool, and the outer peripheral pool is open to the atmospheric air, these two regions are eventually filled with water at the saturation temperature thereof (100.degree. C.), so that the requisite temperature difference cannot be secured between the two regions, resulting in the water circulation being stopped.
With the prior-art technique for increasing heat dissipation presented in the Atomic Energy Society of Japan, the water in the suppression pool is circulated by the convection promoting plate installed in the suppression pool; due to this arrangement, that problem to which no consideration was given in the above prior-art technique can be solved, making it possible to utilize the region below the vent-pipe outlets and attain continuity in circulation. Since, however, only the containment-vessel wall is used as the heat transfer surface for releasing heat from the suppression chamber to the outer peripheral pool, the transfer area can only be enlarged in proportion to enlargement of the high-temperature region of the suppression pool, which means the suppression pool has to be enlarged if a further increase in heat dissipation is desired. That would entail an increase in the size of the reactor containment vessel.
In the prior-art technique described in JP.A.2-227699, in which air cooling is effected, the rate of the heat transfer by the air circulation is lower than that of the convection heat transfer in the pool water, so that a large heat transfer area is needed to attain the requisite heat dissipation characteristic. Further, in this prior-art technique, no consideration is given to the above-mentioned necessity of raising the allowable temperature for the suppression pool.
To attain a further improvement in heat dissipation characteristic in these prior-art reactor containment vessels so that they may be adapted to a nuclear plant of a larger output power, the heat dissipation area might be increased by enlarging the size of the reactor containment vessel. However, such an increase in the size of the reactor containment vessel would be a problem.
In the reactor containment vessels described in JP.A.63-75594 (exclusive of FIG. 4) and JP.A.63-191096, which have been mentioned above, any rupture occurring, for example, in the main steam piping, will, as described above, cause the coolant in the reactor pressure vessel to enter the dry well as steam at high temperature and pressure, which steam will further flow through the vent pipes into the suppression-pool water to condense therein. In this process, part of the coolant in the reactor containment vessel will be drawn down in the dry well. It should be noted here that reactor containment facilities are generally equipped with emergency core cooling systems; when the pressure in the reactor pressure vessel has become lower than a predetermined value, the emergency core cooling system operates to cause water to be fed into the reactor pressure vessel for the purpose of submerging the core. This water further overflows from the rupture opening to be discharged into the dry well, with the result that the water level in the dry well is raised by the drawdown water and this overflow water. When the water level in the dry well has been raised up to the dry-well-side openings of the vent pipes, these hot waters flow into the suppression pool through the vent pipes.
In the above prior-art techniques, however, the positions of the dry-well-side openings of the vent pipes are at high level, so that a large amount of water accumulates in the dry well at the time of an a loss-of-coolant accident as mentioned above; accordingly, the water level in the suppression pool only rises to a small degree, with the result that the contact area with the outer peripheral pool of the containment vessel cannot be greater than a fixed value. Further, since a large amount of hot water accumulates in the dry well, the temperature rise of the suppression-pool water is correspondingly dull and the difference in the temperature thereof and that of the outer-peripheral-pool water is small, with the result that the heat transfer from the suppression pool to the outer peripheral pool only occurs to a small degree. In other words, the cooling capacity of the containment vessel has to remain rather poor.
By lowering the level of the dry-well-side openings of the vent pipes, the amount of water accumulating in the dry well is reduced and the amount of suppression-pool water is augmented, with the heat transfer to the outer pool of the containment vessel increasing. However, depending on the degree to which their level is lowered, it may happen that the dry-well-side openings of the vent pipes are immersed in water. In that case, it is difficult for the water level in the dry well to be smoothly lowered even if the pressure in the dry well is raised by the hot water accumulated therein (Pascal's principle), so that the pressure in the dry well rises to an excessive degree, resulting in the containment vessel being deteriorated in terms of safety.
Thus, it is necessary to ascertain the degree to which the level of the dry-well-side openings of the vent pipes can be lowered without involving any problems.
A prior-art technique for reducing the amount of water accumulated in the dry well is described in JP.A.63-229390, according to which a return line is provided, which extends through the wall in which the vent pipes are formed, i.e., the vent wall, and the opening on the dry well side of this return line is situated higher than the water surface of the suppression pool in the normal condition, whereby the suppression-pool water is prevented from flowing backwards to the dry well. Apart from this, shown in FIG. 4 of JP.A.63-75594, which has been mentioned above, is a structure which includes a core submerging hole allowing the dry well to communicate with the vent-pipes; this core submerging hole is provided in that portion of the vent wall which is on the dry well side, and at a height which is above the normal water level of the suppression pool and which allows the reactor core to be submerged.
When, in these prior-art techniques, core coolant is drawn down into the dry well through any rupture opening, the water level in the dry well rises; when the water level has reached the height of the return line or that of the core submerging hole, the drawdown water flows into the suppression-pool water through the return line or the core submerging hole, thereby preventing the water level in the dry well from being raised. Further, since the drawdown water enters the suppression chamber, the water level of the suppression-pool water rises, thereby increasing the area of the heat transfer surface through which heat is transferred from the suppression chamber to the outer peripheral pool and improving the rate of heat dissipation from the containment vessel, which is required for a medium or long period of time after accident.
In these prior-art techniques, however, the return line or the submerging hole is provided in the vent wall, so that the diameter of the return line or the submerging hole cannot be made large because of the necessity of retaining the requisite level of strength of the vent wall. Therefore, the amount of flow from the dry well to the suppression pool through the return line or the submerging hole is limited, so that, while the water amount in the dry well is increasing at high rate, it is impossible to completely prevent the water-level in the dry well from rising. Thus, the uppermost and hottest portion of the water in the dry well is not transferred to the suppression pool, so that, for a short period after the occurrence of an accident, the containment vessel suffers deterioration in its ability to transfer heat from the dry well to the suppression pool, resulting in the vessel being deteriorated in safety.
Further, there are prior-art techniques in which the reactor core is cooled at the occurrence of a loss-of-coolant accident by supplying water into the core by a static means, as disclosed in "Simplicity; the key improved safety, performance and economics", Nuc. Eng. November 1989, and JP.A.63-229390 mentioned above.
According to the prior-art technique described in Nuc. Eng. November 1989, the cooling of the reactor core for a short period after the occurrence of any loss-of-coolant accident is effected by means of a gravity-driven water pool in an emergency core cooling system, and the cooling of the reactor core for a long period after the occurrence of the same is achieved by returning the pool water in the suppression pool to the pressure vessel through an equalizing system. For this purpose, the equalizing system comprises an equalizing line which connects the suppression-pool water with the pressure vessel, a blasting valve provided in this equalizing line such as to remain closed during normal operation and as to be opened only at the time of an accident, and a check valve for preventing the coolant in the pressure vessel from flowing into the suppression pool.
For a long period after the occurrence of an accident, the water in the containment vessel fills the lower dry well to the full by the gravity-driven water pool, and further fills dry well to the height of the inlets of the vent pipes (or the height of the return line leading to the suppression pool), with drawdown water flowing into the suppression-pool water to raise the water level thereof. As a result, the area of the heat transfer surface through which heat is transferred from the suppression chamber to the outer peripheral pool is augmented, thereby improving the rate of heat dissipation from the containment vessel as required for a medium or long period of time. In this case, it is necessary for the water in the containment vessel to fill the same up to the height of the inlets of the vent pipes (or the height of the return line leading to the suppression pool), with the result that a large amount of gravity-driven-pool water is required.
In the prior-art technique described in JP.A.63-229390, the core cooling for a short period after the occurrence of a loss-of-coolant accident is effected by means of an accumulator water tank provided in the emergency core cooling system, and the core cooling for a long period after the occurrence of the accident is attained by the equalizing system connecting the suppression pool with the pressure vessel, as in the prior-art technique described in Nuc. Eng. November 1989. Also in this case, the water in the containment vessel for a long period in the containment vessel has to fill the dry well up to the height of the return line leading to the suppression pool, so that an accumulator water tank of a large capacity is required.
Thus, in both of the prior-art techniques described in Nuc. Eng. November 1989 and JP.A.63-229390, it is necessary to previously set the water amount of the gravity-driven water pool or the accumulator water tank at a high level, with the result that the wall of the building structure supporting the same must be made thick. In addition, there is the problem that a more strict requirement is imposed on the structure in terms of earthquake-proof property.